1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming a multi-color image by using a plurality of developing devices.
2. Related Background Art
Nowadays, electrophotographic printers have been widely used as an output equipment for a computer, facsimile, CAD and the like. Those printers have been developed to provide a multi-color image. As an example, an electrophotographic printer for forming a two-color image will be explained with reference to FIG. 1. As shown in FIG. 1, after a surface of an electrophotographic photosensitive drum 1 rotating in a direction shown by the arrow, has been uniformly charged by means of a first charger 2, a first electrostatic latent image is formed on the surface of the drum 1 by exposing the drum surface by means of a laser beam 3' modulated in response to first image information. The latent image is then developed with reversal development in a first developing station D1 by means of a first developing device 6 to form a first color toner image. Incidentally, the toner i.e., developer T1 is carried and conveyed by a sleeve 5.
Then, after the drum surface has been charged again with the same charging polarity as the first charger 2, by means of a second charger 7, a second electrostatic latent image is formed on the drum surface by exposing the drum surface by means of a laser beam 8' modulated in response to second image information. The second electrostatic latent image is developed with reversal development in a second developing station D2, by means of a second developing device 10, to form a second color toner image. The charging polarity of the first color toner, used in the first developing device 6, is the same as that of the second color toner, used in the second developing device 10.
The first and second color toner images on the photosensitive drum 1 are transferred altogether onto a transfer sheet 17 by means of a transfer charger 18. The transferred two-color image is fixed to the transfer sheet 17 by means of a fixing device 19; whereas, after transferring operation, the surface of the photosensitive drum 1 is cleaned by a cleaning device 20.
Incidentally, the laser beams 3' and 8' are emitted, respectively, from exposing devices 3 and 8, each comprising a semi-conductor laser and a rotary mirror for deflecting the laser beam emitted from the semi-conductor laser. The photosensitive drum is scanned by such laser beams with a raster pattern. In this example wherein the reversal development is adopted, the laser beam 3' and 8' expose areas to be visualized by the toner. In this example, the first charger and the exposing device 3 constitute a first latent image forming means 4, whereas the second charger 7 and the exposing device 8 constitute a second latent image forming means 9.
The second developing device 13 acts on the drum surface on which the first color toner image has been formed, thereby developing the second latent image. Accordingly, in the second developing station D2, it is so designed that the thickness of a developer layer formed on a sleeve 11 of the second developing device 13 (i.e., a developer bearing member for carrying the developer and conveying it to the developing station) is smaller than a minimum distance between the photo-sensitive drum 1 and the sleeve 11 at the second developing station, whereby the developer carried by the sleeve 11 is prevented from slidingly contacting the first color toner image. Thus, preventing the scraping of the image from the drum.
In order to improve the developing efficiency of the above-mentioned second developing device 10 of a so-called non-contacting type, the alternate bias voltage may be applied to the sleeve, as described in the U.S. Pat. No. 4,292,387. In this case, the developer is moved reciprocably between the photosensitive drum and the sleeve to thereby apply the developer of the amount corresponding to the potential of the latent image onto the drum eventually.
In this case, the alternating bias voltage applied to the sleeve 11 acts to press the first color toner image against the drum 1 in one phase, but to separate the first color toner image from the drum 1 in the other phase. In order to prevent that the first color toner image is separated from the drum 1 and is entered into the second developing device 10 in the other phase, it is preferable that the peak-to-peak value of the alternating bias voltage (absolute value between the maximum value and the minimum value alternately obtained repeatedly) is smaller.
However, if the peak-to-peak value of the alternating bias voltage is small, it is apt to generate a fine powder layer, which is difficult to be flown toward the photosensitive drum, on the surface of the sleeve, thus making the formation of the latent image difficult. The reason is assumed as follows.
That is to say, in the conventional apparatuses, the developing sleeve is normally constituted by a cylinder made of non-magnetic metal such as stainless steel, aluminum or the like. Such material normally forms, on its outer surface, a high resistance oxidized film when it is exposed to the air, and, when the fine particles having large charge and included in the developer contacts with the oxidized film, the fine particles are strongly attracted to the oxidized film due to the charges thereof, thus forming the fine powder layer on the film. Consequently, the toner applied onto the fine powder layer is not frictionally charged adequately between the toner and the sleeve, with the result that the frictional charge of the toner will be insufficient, thereby lowering the flying force of the toner toward the photosensitive drum. For this reason, it is guessed that the density of the image is reduced. Accordingly, in order to compensate such insufficiency, in the conventional apparatuses, it was necessary to increase the peak-to-peak value Vpp of the alternate voltage applied to the sleeve.
However, in the conventional apparatus, since the peak-to-peak value Vpp of the alternating voltage was large, there arose the problems that the first color toner image formed on the photosensitive drum was disturbed and that the toner separated from the first color toner image was attracted into the second developing device to mix with the second color toner.